If you do decide to go with an Acer laptop check Comet online before you buy. I've been looking for one lately and Comet have been far cheaper than anywhere else for certain models. Comet currently have the 5612 model for £599 (the site you linked to has it at £659) which has 1gb ram instead of 512mb and also has an 80Gb hard drive rather than 60gb.
I've also heard that Acer maybe aren't as tough as some other brands but to get another brand with similar spec usually costs a fair bit more.
It's always worth keeping an eye on Dell to see if they have any crazy offers on at the mo. Deals can change weekly so it's worth checking up. Keep an eye on the section that shows you the machines that are being advertised in the press/media as these tend to be the more spectacular deals.
It's a bit of a minefield choosing a laptop at the mo (well, always actually!), good luck !